Nihilego did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Nihilego, known in Japanese as ウツロイド (Utsuroido), is a Rock/Poison-typePokémon, and the first of seven Ultra Beasts. It is also known as UB-01 Symbiont (Japanese: UB01 PARASITE), or simply UB-01.
Nihilego is a medusoid Pokémon with a white, glass-like body and fifteen tentacles: two acting as arms, six long ones flowing like hair (two of which being tipped with a transparent color), two longer transparent ones resembling legs, two shorter ones reminiscent of sideburns and three stubby ones similar to bangs. Despite the jellyfish-like appearance and the lack of a face, Nihilego bears resemblance to a little girl and may even act like one. Its hat-like bell is completely transparent and is adorned with six white stars, each with six points. The brim of its bell is white, large and wavy, and rimmed with a light blue line.
Nihilego is a parasitic Ultra Beast that latches on its host and, rather than manipulating the host directly, injects its host with a neurotoxin that awakens the host's own capabilities and raises those to an extreme extent, in order to protect itself. This special neurotoxin Nihilego possesses is extremely stimulating and inspires extreme excitement in its host, while disabling any sort of inhibition. Anyone that is being latched on by Nihilego and is affected by its neurotoxins will have its native skills forcibly activated to their fullest extent and will act to it as it naturally desires too, often becoming violent. Due to this neurotoxin, Nihilego could be seen as a dangerous creature.
A Nihilego is seen on Aether Island as the main boss of Level 5-5 in KirbiMiroir's world. In 5-6, a swarm of miniature Nihilego, alongside other miniature UBs, attack Aether Island but are ultimately defeated by Lusamine. In the final world, a Nihilego is briefly seen eating the Lusamine Clone and transforming into a giant monster (known in-game as "The Motherbeast") in the process. After its defeat, both to the party and to Lillie's words, The Motherbeast explodes, but somehow leaves both the Nihilego and the now-unconscious Lusamine Clone in the process.
Nihilego is a combination of nihil (Latin for indefinite nothing) and ego.
Utsuroido may be a portmanteau on 虚ろ utsuro (void), void, and id.
Names in other Languages
From 虚ろ utsuro, void and id.
From zéro and id
From ἀν- an- (ancient Greek prefix indicating "not") and ego